The ...spatial.to_featureclass() method no longer overwrites the existing feature class.
Sample code:
import arcpy
import pandas as pd
from arcgis.features import Feature
arcpy.env.overwriteOutput = True
df = pd.DataFrame.spatial.from_featureclass(<valid featureclass>)
#modify features in some fashion
df.spatial.to_featureclass(location=<path to existing feature class>, overwrite=True, sanitize_columns=False)
ArcGIS API for Python version 1.9.1